There are food trucks aplenty here in San Francisco. You will find most have a Twitter account giving you their general whereabouts. There are also lots of roaming 'carts' about the city too. The Tamale Lady, The Creme Brulee Cart, The Magic Curry Kart and others are quite popular... you can either find them on twitter or see who is posting about them.

All this said, I have not visited half as many as I would like. There's the Liba Falafel truck and El Toyanese (usually near the Best Buy) that are two of my favorites (althoug I would avoid the El Toyanese restaurant so you are in luck the truck is good).